Currently, the activated sludge process is used to treat sewage in most municipal sewage treatment plants. The organic substances, virus, bacteria and metal pollutants are transferred from sewage to activated sludge during the process. Residual sludge with a moisture content of more than 99.9% and high contents of organic matter is the mainly waste materials in sewage treatment plants. Treatment and disposal of sewage sludge are a complex problem, and restrict the construction and development of town.
According to the current methods commonly used for treating sewage sludge, the residual sludge with a moisture content of 99% is firstly emptied into a gravitationally concentrating tank. The supernatant is removed after 12-24 hours. The residual sludge is concentrated to have a moisture content of 97%. Then the concentrated sludge is chemically conditioned by cationic polyacrylamide and then emptied into a centrifugal dehydrator, a belt type dryer or a plate-and-frame filter press. The filtrate is removed, leaving the sludge having a moisture content of 80-75%. The dehydrated sludge is pre-dispersed to particles and stripes and then dried in a dryer to get the dried sludge with a moisture content of 40-30%. The dried sludge may be further landfill disposed, incinerated or reclaimed as cements, bricks, fertilizers for garden and low heat value fuels.
However, the commonly used methods for treating sludge have the following disadvantages. (1) The efficiency for the concentration of sludge is low. It takes more than 12 hours for gravitationally concentrating, leading to the huge volume of the concentrating pool and rottenness of sludge during the concentration process. (2) The efficiency for the dehydration of sludge is low. The dehydrated sludge is in huge quantity and has high moisture content. Accordingly, the quantity of moisture which is needed to evaporate in the subsequent heat treatment is huge, leading to high investment and operating costs for the heat treating equipment. (3) The dehydrated sludge with a moisture content of 80-75% is semisolid with a high viscosity and thereby hard to disperse during drying process. It is known that the sludge in the viscose phase (with a moisture content of 60-40%) has low heat and mass transferring efficiency. When drying, such sludge requires high energy consumption and is subjected to “soft core phenomenon” which is dry on surface but wet in interior.